The present invention relates to a rotational angle detector for detecting the angle of a rotational body, and a method for initializing a rotational angle detector.
Various systems for improving traveling stability such as a vehicle stability control system and an electronic control suspension system are being installed in recent vehicles, which have become highly sophisticated. These systems acquire the steering angle of a steering wheel as attitude information of the vehicle and control the vehicle so that the attitude of the vehicle is maintained in a stable state based on the attitude information. Accordingly, a rotational angle detector for detecting the steering angle of the steering wheel is incorporated in, for example, a steering column of the vehicle. Such type of a rotational angle detector employs an absolute angle detection technique, which detects the steering angle with an absolute value, or a relative angle detection technique, which detects the steering angle with a relative value. The detection technique that is employed is determined in accordance with the product specification and the like.
Japanese Laid-Open Patent Publication No. 2004-309222 describes an example of a rotational angle detector adopting the absolute angle detection technique. The rotational angle detector includes a drive gear that integrally rotates with the steering shaft, and two driven gears mated with the drive gear. A magnet is arranged on each driven gear to rotate integrally with the driven gear. The two driven gears have a different number of teeth. Thus, the two driven gears are rotated at different rotational angles by the rotation of the drive gear. The rotational angle detector has a controller that determines the rotational angles of the two driven gears with the corresponding magnetic sensors to obtain the rotational angle of the steering shaft.
However, this conventional rotational angle detector has the following problems. The drive gear is mated with the two driven gears. Thus, the detected rotational angles of the two driven gears contain errors resulting from backlash between the drive gear and the two driven gears. Accordingly, the rotational angle of the steering shaft obtained from the rotational angles of the two driven gears potentially contains an error (initial steering error). The vehicle system thus corrects the error in the rotational angle of the steering shaft and computes the true rotational angle of the steering shaft. In this case, however, a great amount of correction data for the rotational angle of the steering shaft is necessary. Further, the computation amount for the correction with the correction data is large. Such a correction process may be performed with the vehicle system. However, this would increase the computation load on the vehicle system.
The amount of the backlash between the drive gear and the driven gear differs when the drive gear rotates in the left direction and the right direction depending on the positional relationship between the drive gear and the two driven gears. In this case, the error (steering angle error) of the rotational angle of the steering shaft caused by a bias in the backlash also differs between left and right rotations. When taking into consideration that the error in the rotational angle may differ between left and right rotations of the steering shaft, the computation load on the rotational angle detector or on the vehicle system side of the vehicle further increases.